Background and Objective: Obesity is a one of the major problems of developing societies and its prevalence is steadily increasing. This research aimed to compare the effectiveness of cognitive behavior therapy and diet therapy on weight reduction and promotion of lifestyle self-efficacy in obese people.Methods: This was a semi-experimental research with a pre-test and post-test design. From the obese people referring to psychological counseling centers and nutrition clinics of Tehran city, 30 people 20 to 40 years old were selected by available sampling method and randomly assigned to two groups. The groups were trained in 10 sessions (one session per week) and body mass index and Clark et al’s lifestyle self-efficacy were measured in pre-test and post-test phases. The data were analyzed by T-Test and MANCOVA methods.Results: The findings showed that both two methods of cognitive behavior therapy and diet therapy were effective on weight reduction and promotion of lifestyle self-efficacy in obese people. Also cognitive behavior therapy in comparison diet therapy significantly led to weight reduction and promotion of lifestyle self-efficacy of obese people (P<0/001).Conclusion: The results indicated the importance of both two methods especially cognitive behavior therapy method in weight reduction and promotion of lifestyle self-efficacy of obese people. Therefore, both two methods can be used by counselors and therapists in order for weight reduction and promotion of lifestyle self-efficacy of obese people.